[XorNot]

The system isn't closed.

You're functionally drawing solar energy off the system very inefficiently (if you wanted kinetic motion).

A different way to look at the problem is that you can't have water spontaneously move up hill, but if you dam a river you can absolutely extract useful energy from it.

A turbine underneath the ocean could extract energy from ocean currents and this is the same problem.

[shiandow]

So according to you you could simply lower a membrane and a pipe into a still body of salt water and it would spontaneously separate into sweet water and brine until the build up of brine prevented this from continuing?

Yeah I am still not seeing it. If that were the thermal equilibrium I don't see how it wouldn't separate spontaneously, or why you can mix salt and water with no input of energy whatsoever.

It goes against anything I know about entropy and osmotic pressure.

[XorNot]

At this point you are simply arguing reverse osmosis is impossible. There is no functional difference between mechanically creating a pressure differential across the membrane with a pump, and lowering a membrane deep enough that the pressure differential can drive the process.

[shiandow]

Reverse osmosis without work is indeed impossible.